In lithium-ion batteries cathodes typically comprise a lithium-containing material and the anode is usually a lithium-free material such as graphite, a metal, a metalloid, or an oxide. In lithium-sulfur batteries, the anode is typically lithium metal and the cathode is made of sulfur or a carbon/sulfur composite. New lithium-sulfur battery devices use a metal/carbon composite as an anode and a polysulfide cathode (e.g. Li2S). In lithium ion batteries, the anode may be: i) a metal or a metalloid that can alloy with lithium, mainly elements form groups: 2B (Zn and Cd), 3A (Al, Ga and In), 4A (Si, Sn and Pb), 5A (Sb and Bi) and Sn-alloys (Sn—Fe, Sn—Co, Sn—Ni, Sn—Cu, Sn—Zn); ii) hard or soft carbon (e.g. graphite), iii) an oxide whose metal allows with lithium such as SnO2, Sb2O3, and silicon oxide); iv) a transition metal oxide (Li4Ti5O12, titanium oxide, chromium oxide, manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxide and zinc oxide). Lithium also intercalates into nitrides, phosphides and sulfides. In lithium-ion batteries, of particular importance are anodes in which lithium is intercalated into an oxide or an oxide-carbon matrix.
Tin oxide reacts with lithium according to the following reaction:SnO2+4Li++4e−→Sn+2Li2O irreversible reaction (711 mAh/g)Sn+4.4Li++4.4e−→SnLi4.4 reversible reaction (783 mAh/g)
Anodes that comprise metal (or a carbon/metal composite or metal oxide) that alloys with lithium suffer from large volume change upon lithiation/delithiation of the electrode during operation and re-charging of the battery. This volume expansion ranges from about 100% for Al to about 300% for Si. In order to accommodate this large volume change it is necessary either to use nanoparticles or use a binder that can accommodate this volume change. Polyvinylidene fluoride (PVDF) is the conventionally used binder in battery technology; however, it does not accommodate a volume change larger than about 15-20%, such as for graphite or Li4Ti6O12. PVDF does not get reduced at low potential (5 mV versus Li/Li+) nor oxidized at high potential (5 V versus Li/Li+) at room temperature. However, at elevated temperature, it has been reported that PVDF reacts with Li metal and LiC6 to form LiF and some C═CF species via an exothermic reaction which will cause a risk for thermal runaway (Du Pasquier 1998; Maleki 1999; Maleki 2000). To avoid this risk, research has focused on the use of non-fluorinated binders (Gaberscek 2000; Oskam 1999; Ohta 2001; Zhang 2002; Verbrugge 2003). Even though they are still insoluble in water, reduced heat was obtained when phenol-formaldehyde, poly(vinylchloride) or polyacrylonitrile are used as binders (Maleki 2000; Du Pasquier 1998). Another disadvantage of using PVDF is its price which is about US $20 per kg in North America (ε15-18 per kg in Europe (Lux 2010)). In addition, PVDF requires the use of non-environmentally friendly solvents to process the electrode formation, such as N-methyl-2-pyrrolidone (NMP). Also, it is not easy to dispose of PVDF at the end of the battery life (Lux 2003). Thus more environmentally friendly binders are needed for preparing electrode materials for Li-ion batteries.
Some rubber-based binders such as styrene-butadiene rubber have been tested with some success, but these binders are not water soluble and there is a further need to improve their ability to accommodate volume expansion. Sodium carboxymethylcellulose (NaCMC) is a sugar-based molecule used as a thickener in the food industry and has shown good accommodation of volume expansion in the case of silicon-based electrodes (Li 2007; Buqa 2006; Beattie 2008; Hochgatterer 2008; Liu 2005), and more recently with tin oxide-based electrodes (Chou 2010). In addition to being able to accommodate the volume change, NaCMC is water soluble due to the carboxymethyl groups attached to the cellulose. This avoids the use of non-environmentally friendly solvents during the casting process, which makes fabrication of the electrode easier. As mentioned by Lux et al., the use of NaCMC also makes the recycling of Li-ion battery anodes easier (Lux 2010). Indeed, by heating NaCMC at 700° C., Na2CO3 is obtained. In addition (Machado 2003; Kaloustian 1997), the price of NaCMC is much lower than PVDF, about US $6 per kg in North America (ε1-2 per kg in Europe (Lux 2010)).
One approach in the prior art (Satoh 2005; Satoh 2008; Satoh 2009a; Satoh 2009b) for producing negative electrodes for batteries has been to use graphite-based anodes and binders having olefinic unsaturated bonds (e.g. styrene-butadiene rubbers). In such an approach, the graphite-based anode may be coated with metal oxides. This prior art also suggests that xanthan gum may be used as a co-binder along with the binder having olefinic unsaturated bonds. However, there has been no specific exemplification of the use of xanthan gum and the use of xanthan gum exclusively, i.e. not as a co-binder, has not been suggested. Further, the anode material is restricted to graphite or graphite coated on the surface with metal oxides. Such anode materials do not experience the very large volume expansions that oxide or oxide-carbon matrix materials undergo during the lithiation/delithiation process.
There remains a need in the art for water soluble binders which can accommodate large volume expansions upon lithiation/delithiation of electrodes in lithium-ion or lithium-sulfur batteries.